1. Field of the Invention
The present invention relates to a photosensitive member for electrophotography used in electrophotographic devices such as copying apparatuses, printers and facsimiles in which a coherent light such as laser light is used for exposure.
2. Description of the Prior Art
The photosensitive members for electrophotography must have high sensitivity, long life and high heat resistance and provide images of high quality over a long time. Recently, there have been put to practical use electrophotographic devices such as copying apparatuses, printers and facsimiles in which images are formed by optically scanning a photosensitive member with a laser light modulated depending on digital image information to expose the photosensitive member. In these electrophotographic devices, small-sized semiconductor lasers have widely been employed as laser light sources. The photosensitive members used in such apparatuses must be highly sensitive to semiconductor laser beam whose wavelength ranges from 750 to 800 nm.
As such photosensitive members for electrophotography, there have been known separated-function photosensitive members in which selenium-containing materials are used as photoconductive materials and which comprise a substrate provided thereon 3 to 4 laminated photosensitive layers. The separated-function photosensitive members are detailed in, for instance, U.S. Pat. No. 3,655,377 and Japanese Patent Application Laying-Open Nos. 4240/1977 and 77744/1980. The photosensitive members basically have a structure which comprises, as shown in FIG. 1, a conductive substrate 1 provided thereon with in order a charge transport layer (CTL) 2, a charge generation layer (CGL) 3 and an overcoating layer (OCL) 4.
The separated-function photosensitive member provided with such laminated photosensitive layers does not provide high quality images since a laser light used for exposing the member causes multiple reflection in the photosensitive layer thereof and as a result, and interference pattern is formed on the resulting images, in particular half-tone dot images.
FIG. 2 is a schematic diagram for illustrating the multiple reflection observed when a conventional separated-function photosensitive member which is commonly utilized and has a structure shown in FIG. 1 is irradiated with a laser light. FIG. 2 illustrates the multiple reflection which is generated by a laser light diagonally incident upon the surface of the conventional separated-function photosensitive member in order to explain obviously a reflection at an interface between CGL 3 and OCL 4. A part of a laser light 6 incident upon the surface of the photosensitive member is reflected at the interface between the air and OCL 4 due to the difference between the refractive indexes thereof, while the remaining laser light I, pass through OCL 4 and is made incident upon CGL 3. At this stage, a part of the laser I, is likewise reflected at the interface between CGL 3 and OCL 4 because of the difference between the refractive indexes thereof, but a reflected light is partially reflected at the interface between the air and OCL 4 to form a reflected light light R.sub.1 which passes through OCL 4 and partially reflected at the interface between CGL 3 and OCL 4 and the remaining light light is made incident upon CGL 3. A part of the light R.sub.1 reflected at the interface between CGL 3 and OCL 4 is further partially reflected at the interface between the air and OCL 4 to form a reflected light light R.sub.2 which, like the light R.sub.1, passes through OCL 4 and partially reflected at the interface between CGL 3 and OCL 4 and the remaining light is made incident upon CGL 3. As has been discussed above, the incident light reflected at the interface between CGL 3 and OCL 4 undergo multiple reflection within OCL 4 and are made incident upon CGL 3 and the light 8 incident upon CGL 3 are interference lights of I.sub.1, R.sub.1, R.sub.2 . . . . Thus, the intensity of the laser lights vertically incident upon CGL 3 varies dependent upon the thickness of OCL 4 and if the refractive index of OCL 4, the deviation in the film thickness thereof and the wavelength of the laser light is defined to be n.sub.1, d and .lambda., respectively, the number (m) of the interference fringes generated is equal to 2n.sub.1 d/.lambda..
The laser light vertically incident upon the photosensitive layer undergoes multiple reflection within the overcoating layer (OCL) 4 and the reflected light causes interference because of the coherency of the laser light. In this case, if OCL 4 has a deviation in film thickness, the reflected light is intensified or weakened due to the interference and correspondingly the number of charges generated within a charge generation (CGL) 3 is increased or decreased. As a result, such an interference pattern is formed. Further, the laser light which is not absorbed by CGL 3 and transmitted therethrough reaches the surface of conductive substrate 1, causes regular reflection and the reflected light causes multiple reflection within CTL 2 to thus cause interference. In this case, if CTL 2 has a deviation in film thickness, the reflected light is intensified or weakened due to the interference as has explained above in connection with OCL 4 and this likewise becomes a cause of the appearance of the interference pattern. The interference patterns due to these phenomena which are superimposed to one another often appear on the resulting image.
There have been known methods for eliminating the latter interference pattern resulting from the light which transmits through CGL and is reflected by the surface of the substrate by treating the surface of the substrate. These methods are detailed in, for instance, Japanese Patent Application Laying-Open Nos. 225854/1985, 254168/1985 and 167761/1989. However, there has not yet been developed any means for eliminating the former interference pattern resulting from the light directly incident upon CGL.
As has been described above, the selenium type separated-function photosensitive member has a basic structure as shown in FIG. 1. In the case of photosensitive member having such a structure, the laser light directly incident upon the member greatly contribute to the generation of charges as compared with the contribution by the laser light which transmits through CGL and is reflected by the surface of the substrate. For this reason, the interference pattern resulting from the interference due to the multiple reflection, within OCL, of the laser light incident upon the photosensitive member would be formed easier than that resulting from the interference due to the multiple reflection, within CTL, of the laser light reflected by the substrate surface. Under such circumstances, the inventors of this invention have supposed that it is necessarily more important and effective to eliminate the effect of the former interference than to eliminate the influence of the latter interference, in order to solve the problem of the formation of interference patterns.